The present invention relates to a multi-channel stereo converter, comprising stereo means for generating an information signal from stereophonic audio signals (L, R) and transforming means coupled to the stereo means for transforming said audio signals (L, R) to a further audio signal (C; S).
The present invention also relates to a method for generating audio signals from stereophonic audio signals (L, R), wherein an information signal is derived from said audio signals (L, R) and used for transforming said audio signals (L, R) to such an audio signal (S).
Such a multi-channel stereo system and method are known from U.S. Pat. No. 5,426,702. The known system comprises stereo means in the form of a direction detection circuit for generating an information signal, which is derived from stereophonic audio input signals (L, R). The information signal contains a weighting factor measure for the direction of a most powerful sound source. Furthermore the known converter system comprises transforming means coupled to the direction detection circuit for transforming said audio signals (L, R) to a further audio signal in the form of an audio centre signal.
It is a disadvantage of the known multi-channel converter and method that no provisions are made to generate surround audio signals.
Therefore it is an object of the present invention to provide a multi-channel stereo converter system and corresponding method capable of generating and handling a variety of auxiliary audio signals, such as surround, stereo surround and/or centre signals, without substantial cross talk between these auxiliary audio signals.
Thereto the multi-channel stereo converter according to the invention is characterized in that the stereo means are stereo magnitude determining means for generating a stereo information signal (a/b; xcfx81), which represents a degree of stereo between said audio signals (L, R), and that the transforming means are embodied for transforming said audio signals (L, R) based on said stereo information signal (a/b; xcfx81) into at least a surround signal (S).
Similarly the method according to the invention is characterized in that the information signal is a stereo information signal (a/b; xcfx81), which represents a degree of stereo between said audio signals (L, R), and that based on said stereo information signal (a/b; xcfx81) said audio signals (L, R) are transformed into at least a surround signal (S).
It is an advantage of the multi-channel stereo converter and method according to the present invention that it is capable of generating additional related audio signals, such as surround signals, and left and right stereo surround signals, and/or at wish an audio centre signal, based on the two stereophonic left (L) and right (R) audio signals. This gives a large degree of freedom both in application possibilities and design, without substantial cross talk between output audio signals.
An embodiment of the stereo converter according to the present invention characterized in that the transforming means use a relation for said transformation which maps the stereo information signal (a/b; xcfx81) to an angle (xcex2) onto an audio signals defined plane. In a very simple to implement embodiment said transformation uses a goniometric relation. In practice one would consider the use of some transformation which maps the stereo information signal (a/b; xcfx81) to the angle (xcex2), where this angle is between 0 and xcfx80/2.
A particular embodiment of the multi-channel stereo converter according to the invention is characterized in that the transforming means are embodied for additionally transforming said audio signals (L, R) from an orthogonal representation to a representation, wherein said audio signals (L, R) lie on a straight line, thus revealing an additional audio centre signal (C).
Advantageously this embodiment provides for a multi-channel configuration having available audio left (L), right (R), surround (S) or surround left (SL) and surround right (SR), and the audio centre signal (C).
Advantageously a vector multiplication with a multiple which lies around two, can in particular with a matrix transformation be implemented easily on chip. In a further embodiment of the multi-channel stereo converter according to the invention matrix coefficients of said matrix transformation are based on projections of an actual audio signal on principal axes of the audio signals (R, L, C, S), either or not combined with other coefficients, such as empirically determined coefficients, to cover for example Dolby(copyright) Surround, Dolby Pro Logic(copyright), Circle Surround(copyright), and Lexicon(copyright) systems and other surround systems.
In practice a still further embodiment of the multi-channel stereo converter according to the invention is characterized in that the stereo converter is provided with one or more decorrelation filters, for example Lauridsen decorrelation filters, to which filters the stereo surround signal (S) are applied for generating a stereo surround left signal (SL) and a stereo surround right signal (SR). These kind of decorrelation filters are readily available on the market.